This invention relates to ignition systems for spark-ignited internal combustion engines and more particularly to apparatus for electronically modifying spark advance and dwell in a spark-ignited internal combustion engine.
The ignition system of a conventional spark-ignited internal combustion engine includes a set of breaker points mounted in a distributor and an ignition coil. The breaker points are periodically closed to allow a current buildup in the primary winding of the ignition coil. An engine driven cam then opens the breaker points and the collapsing magnetic field in the core of the ignition coil establishes a high secondary voltage for firing a spark plug to ignite a compressed fuel-air mixture. The spark is timed to ignite the compressed fuel-air mixture at a predetermined position on the compression stroke. The degree of spark advance or retard is determined by various factors, some of which include engine design, engine speed, engine load, gas quality and exhaust emission standards. In a typical automobile engine, the spark advance is determined by three separate factors: the mechanical setting or orientation between the breaker points and the engine driven cam, a centrifugal advance which increases the spark advance as engine speed increases and a vacuum advance operated in response to the vacuum within the intake manifold on the engine. Another important factor in tuning an internal combustion engine is the dwell of the breaker points. Dwell may be defined as the ratio of he time that the breaker points are closed to the total time of an ignition cycle and is generally referenced as degrees in crankshaft rotation during which the breaker points are closed. The dwell affects the available voltage for firing the spark plugs and is most important at higher engine speeds where the breaker points are closed for a minimum time.
In the conventional spark-ignited internal combustion engine, spark advance and dwell must be mechanically set. The dwell is normally changed by adjusting the maximum gap in the breaker points. In many cases, the engine must be stopped and a cap must be removed from the distributor in which the breaker points are mounted for adjusting dwell. After dwell is adjusted, spark advance is set by physically rotating the distributor housing with respect to the engine driven camshaft which operates the breaker points. Any time the dwell is changed, the spark advance will be affected and therefore must be readjusted. It is sometimes desirable to have the capability of easily changing the spark advance and dwell for an internal combustion engine while such engine is running. For example, there is sometimes a need to easily modify the spark advance and dwell on a running engine for studying or demonstrating the effects of spark advance and dwell on conditions such as engine output power and engine exhaust emissions. However, the ignition system in a conventional internal combustion engine does not lend itself to easily modifying the spark advance and dwell while the engine is running or to modifying the dwell without affecting the spark advance.